The STS system is a FCC function that when engaged calculates the climb air speed (trimmed speed), external conditions like wind speed and direction affect air speed, the FCC moves the stabilizer to hold its calculated climb speed (provided the pilot is not moving the stab or auto pilot is not engaged), if air speed increases the only way the FCC can slow down the aircraft is to increase drag by moving the stab areoplane nose up or if air speed decreases the only way the FCC can speed up the aircraft is to decrease drag by moving the stab areoplane nose down. FCC reaction time are very low so this should make it easier for the pilot to maintain that constant climb speed. Boeing believe that once the climb speed is set by the pilot is should remain constant.
The system in not about the pilot changing the speed (it will oppose pilot speed change if the stab is not used). Boeing believes the if the pilot wants to change speed they will operate the stab trim switches (disabling STS) 5 seconds after release of the trim switches FCC will recalculate climb speed and trim accordingly.
All take off’s, climbs and go arounds are high thrust according to the FCC.
The stab should not move automatically until 10 seconds after takeoff.
The STS is there to cover all calibers of pilot.